The Wellcome Trust/Cancer Research UK Gurdon Institute and Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.The Wellcome Trust Sanger Institute, Hinxton, Cambridge, United Kingdom.

Article Figures & Data

Figures

Table showing predominant sensors, signaling, and effector proteins for major DNA repair pathways. Pathways of DSB repair are in blue-shaded area; pathways of SSB repair are in red-shaded area. Main targets of drug development are in red (see the text for details).

DNA DSB repair signaling pathways through the apical DDR kinases. A, DNA-PK: Ku binds to DNA DSBs and recruits DNA-PKcs. Upon DNA binding, autophosphorylation of DNA-PKcs induces a conformational change that destabilizes the NHEJ core complex, causing sliding of Ku inward on the DNA and enabling access of end-processing and ligation enzymes to DNA ends and facilitation of repair. B, ATM: following DSBs ATM is predominantly activated through interactions with NBS1 of the MRN complex. ATM is the principal kinase responsible for phosphorylation of histone H2AX on serine 139 (known as γH2AX). MDC1 (mediator of DNA-damage checkpoint protein 1) directly binds γH2AX and potentiates DNA-damage signaling leading to spreading of γH2AX to over a megabase from its initial lesion. This in turn promotes recruitment and retention of DNA-damage mediator proteins such as 53BP1. CHK2 is a well-studied ATM substrate. C, ATR: ATR is activated by replication protein A (RPA) bound to ssDNA. The ATR–CHK1 signaling cascade activates the G2–M checkpoint, promotes replication fork stabilization, and slows DNA replication by suppressing origin firing.

DNA end processing and DNA polymerase action may be required before ligation can occur, making NHEJ inherently error prone.

NHEJ maintains genome stability, however, by rapidly repairing DSBs in circumstances where recombinogenic events would likely result in gross chromosomal rearrangements; in noncycling or G1 cells, for example (38, 39).

Homology-directed repair

Homologous recombination (HR)

Relatively slow and restricted to late-S phase/G2, as it generally relies on a homologous sister chromatid DNA strand for repair.

Extensive DNA end resection by helicases and exonucleases, such as DNA2, BLM, WRN, and EXO1, results in a 3′–ssDNA overhang, committing the break to repair by HR.

Replication protein A (RPA) coats and stabilizes the ssDNA, leading to ATR activation and subsequent signaling events.

BRCA2, with the help of BRCA1 and PALB2, loads RAD51 onto the RPA-coated ssDNA, leading to strand invasion, with a number of factors negatively regulating this process to prevent hyper-recombination such as POLQ, PARI, RECQL5, FANCJ, and BLM (151).

Alternative (Alt)-NHEJ or MMEJ

Ligation pathway for DSBs when c-NHEJ is genetically compromised (152).

Occurs following limited DNA end resection.

Contributes to the excessive genomic deletions and chromosomal translocations seen in tumors and may also provide a back-up repair pathway in HR-deficient cells (10, 20).

Single-strand annealing (SSA)

Mutagenic, RAD51-independent repair pathway, involving annealing of short or longer complimentary DNA sequences on resected DNA with subsequent deletion of the intervening DNA sequence. The detailed mechanism has yet to be defined in mammalian cells (20).

Involves removal of a short oligonucleotide, including the damaged lesion using structure-specific endonucleases and subsequent restoration of the DNA sequence by DNA polymerases (156).

Mismatch repair (MMR)

MSH2, MSH3, and MSH6 recognize base–base mismatches and insertion/deletion loops, where they recruit MLH1 and PMS2 to damaged sites. The concerted actions of the MMR proteins engage EXO1 to remove the mismatch and then POLD and LIG1 to fill the gap and seal the nick, respectively (157).

Table 2.

Monotherapy and combination trials involving DDR inhibitors that have completed or are currently recruiting (www.clinicaltrials.gov)